When fossil fuels such as petroleum or coal are combusted, sulfur oxides (SOx), nitrogen oxides (NOx), and particulates such as sulfates, soot (carbon; graphite), or soluble organic fractions (SOF) derived from a sulfur content in the fossil fuel are generated and emitted to the atmosphere together with an exhaust gas. For example, the sulfur oxides have significant adverse effects on the global environment, such as causing acid rain or the like, destroying a nature environment such as a forest or a lake, and having a large impact on the ecosystem.
In such circumstances, for example in the automobile industry, technologies for exhaust gas control have been vigorously developed, such as a combination of high-pressure injection and exhaust gas recirculation (EGR), homogeneous charge intelligent multiple injection, a novel NOx catalyst. In addition, in the petroleum refining industry, there is a need to further reduce a concentration of the sulfur content in gasoline or gas oil with a view to applying the EGR effective for reducing NOx and reducing an impact on an after-treatment apparatus for removing the particulates.
From such viewpoints, a regulation particularly on the sulfur content in a petroleum fraction (hydrocarbon oil) such as gasoline, kerosene or gas oil, or heavy oil has been tightened, and a catalyst for hydrogenation treatment having excellent desulfurization activity enabling more efficient removal of the sulfur content in a hydrocarbon oil has been developed.
As a catalyst for hydrogenation treatment industrially currently used for removing sulfur in apetroleum fraction, a catalyst obtained by supporting a periodic table group VI metal such as molybdenum and tungsten and a periodic table group VIII to X metal such as cobalt and nickel on a porous alumina support is typical. As a catalyst having additionally excellent desulfurization activity, a hydrodesulfurization catalyst using a porous titania support has been known.
However, in fact, such catalyst is hardly industrially used as the catalyst for hydrogenation treatment for the purpose of removing the sulfur content in a hydrocarbon oil, so-called desulfurization, because titania has shortcomings such as a small specific surface area, poor formability, and low mechanical strength as compared to alumina, and further, is economically disadvantageous owing to its higher raw material cost as compared to alumina.
In view of the foregoing, various studies have hitherto been made for overcoming such shortcomings of the titania support.
For example, in Patent Literature 1, there is obtained a high-performance hydrodesulfurization catalyst having excellent thermal stability, a large specific surface area, high dispersion of a catalyst metal, improved catalytic activity, and high mechanical strength, by adding as a particle growth inhibitor an anion and a cation to a hydrosol or hydrogel of a hydrous oxide of titanium produced by a pH swing method, or a dried substance thereof, followed by drying and calcination.
In addition, for the purpose of obtaining a catalyst excellent in economic efficiency as well as desulfurization activity and mechanical strength, there has been made an attempt to make a composite of alumina having a low raw material cost and titania promising high performance. In Patent Literature 2, there is proposed a technology of a method of producing a catalyst support for hydrorefining treatment, involving co-precipitating an aluminum ion and a titanium ion to make a composite. In addition, in Patent Literature 3, there is proposed a method of producing an alumina-titania composite catalyst support, involving adding a titanium hydroxycarboxylate and/or a sol of titanium oxide or titanium hydroxide and a hydroxycarboxylic acid to aluminum oxide and/or aluminum hydroxide, followed by mixing and kneading, and calcination. Further, in Patent Literature 4, there is proposed a method of apparently converting a pore surface of an alumina support to titania, involving introducing a titanium tetrachloride gas to the alumina support to perform chemical vapor deposition of titanium on a surface of alumina. Further, in Patent Literature 5, there is proposed a technology of coating a pore surface of alumina with titanium, involving impregnating an alumina support with a solution containing titanium, followed by drying.
Further, for example in Non Patent Literature 1, there is disclosed a method of obtaining a titania-alumina support, involving precipitating (coating) titanium hydroxide on a surface of alumina hydrate particles, followed by aging, filtration, washing, forming, and calcination. In addition, the inventors of the present invention have proposed a catalyst production technology capable of producing a catalyst having a large specific surface area and high mechanical strength, and exhibiting activity as the hydrodesulfurization catalyst comparable to that of the hydrodesulfurization titania catalyst, even when 13 mass % or more of titanium oxide is supported, involving supporting titanium oxide on a surface of an inorganic oxide through precipitation and lamination of titanium oxide between an isoelectric point of the inorganic oxide and an isoelectric point of the titanium oxide, to chemically and macroscopically integrate the inorganic oxide and the titanium oxide (Patent Literature 6).
Further, in Patent Literature 7, there is disclosed a method of producing a catalyst for hydrogenation treatment, involving impregnating an alumina support with a catalyst component-containing aqueous solution containing a catalyst metal, phosphoric acid, and an additive selected from a dihydric or trihydric alcohol having 2 to 10 carbon atoms in one molecule, an ether thereof, a monosaccharide, a disaccharide, and a polysaccharide, followed by drying at 200° C. or less. In addition, in Patent Literature 8, there is proposed a method involving adding a water-soluble organic compound having a molecular weight of 100 or more and having a hydroxyl group and/or an ether bond, such as a diol, an alcohol, an ether group-containing water-soluble polymer, a saccharide, and a polysaccharide, to a catalyst component-containing aqueous solution, in production of a catalyst for hydrogenation treatment by supporting a catalyst metal on a support obtained by supporting an aqueous solution containing a titanium compound on an alumina hydrogel and then performing calcination. Further, the inventors have proposed a method of obtaining a titania catalyst for hydrogenation treatment, involving coating a surface of alumina hydrate particles with titanium hydroxide particles, followed by forming and then drying, and impregnating an obtained titania-coated alumina support with a catalyst component-containing aqueous solution containing a catalyst metal compound and a saccharide, followed by drying (Patent Literature 9).
Although those titania catalysts for hydrogenation treatment using titania supports exploit excellent features of a titania support and overcome the shortcomings to some extent, the problem of being particularly economically disadvantageous from an industrial viewpoint has not yet been overcome.
Meanwhile, for the catalyst for hydrogenation treatment, there has been made an attempt to reactivate a used catalyst that has been used for hydrogenation treatment of a hydrocarbon oil and thus has reduced catalytic activity and utilize the catalyst as a regenerated catalyst.
For example, in association with a catalyst for hydrogenation treatment obtained by supporting a catalyst metal on an inorganic oxide support containing alumina and titania, there is disclosed, in Patent Literature 10, using, in a second desulfurization step, a regenerated catalyst regenerated through a precipitated coke removal reaction under the conditions of an air partial pressure of from 0.05 to 5 MPa and a temperature of from 200 to 800° C., in the case of conducting hydrogenation treatment of gas oil by two steps of a first desulfurization step and the second desulfurization step. In addition, in association with the above-mentioned catalyst for hydrogenation treatment, there is disclosed, in Patent Literature 11, regenerating a used catalyst through calcination treatment at 300° C. for 1 hour in a nitrogen atmosphere, followed by calcination treatment at 450° C. for 3 hours in a mixed gas atmosphere of 50% nitrogen gas and 50% air.
Further, in association with a catalyst for hydrogenation treatment obtained by supporting a catalyst metal such as molybdenum, cobalt, nickel, and phosphorus on an alumina support containing in the catalyst an organic additive such as diethylene glycol, citric acid, and polyethylene glycol, there is disclosed, in Patent Literature 12, performing stripping treatment at from 100 to 370° C. in the presence of an oxygen-containing gas, and then, performing regeneration treatment at from 300 to 500° C. in the presence of an oxygen-containing gas. In addition, in association with the above-mentioned catalyst for hydrogenation treatment, there is disclosed, in Patent Literature 13, activating a used catalyst through regeneration treatment at from 300 to 650° C. in the presence of an oxygen-containing gas after stripping treatment of hydrocarbon, followed by impregnation with “a solution containing an acid and an organic additive” such as: citric acid and polyethylene glycol; or phosphoric acid and polyethylene glycol, and then drying.